`Digraph' means two marks.  This definition  doesn't help much.
There are certainly two marks involved in any letter with a diacritic, e.g., é.
Some times there are two marks required for a conventional letter such as the dots on [j] and [i].
So either we say that both æ and é are digraphs or that as long as they take only one letter space, neither are. 

Although æ is a character in IPA [the intenational phonetic alphabet] and in Icelandic, it is also clearly a ligatured A and E.  Accented letters are not ligatures.  A ligature combines two known symbols into a single letter.  Printer's ligatures do not change the sound values of the component letters.  In a notation, the ligatures are clearly meant to be associated with a distinct sound value related to but not reduced to the two component letters. æ in IPA is not the same æ ae in ITA or Icelandic.  In each case it references a different sound or has a different sound value.

It is certainly possible to replace /ei/ with é .The French writing system allows this.
Consider the name of the swiss alpine lake written in French . . .
.

Lac Léman.[laak ley-maan]  or  [laak ley-maaⁿ] or ...?

Writing systems that have diacritics usually use them in two ways.  This sometimes makes interpretation ambiguous. The é above could be interpreted as showing stress or it could be interpreted as clarifying the sound of the letter [e instead of e].  In the case of  Lac Léman, it is the latter.

English speakers might transcribe Lac Léman as  "lack lemon" since in English stress is generally placed on the initial root syllable and the vowel in the second syllable is converted into a schwa. /ar/ is also the sound most commonly associated with the letter a - although it is not the most frequent.

Nolij that the name of the Swiss lake is written in French would suggests two things:  (1) the stress is on the final syllable and (2) the final consonant is not pronounced but used to indicate nasalization.  Thus the second transcription above [laak ley-maan] is correct as long as the N is interpreted as a marker rather than a terminal consonant. 

Shavian sound-symbol-keyboard table

Graphic with the vowel trapazoid, IPA chart, and SAMPA vowels.
 

SAMPA .ASCII IPA Vowels  http://www.phon.ucl.ac.uk/home/sampa/home.htm

Sp  Sampa  ASCII    Key     IPA  symbol             Description 

 aa    A      65    alms    script a        open back unrounded, Cardinal 5, Eng. start

 a.    {     123     ax      æ ligature      near-open front unrounded, Eng. trap

 'a     6      54     -       turned a          open schwa, Ger. besser       

 o.    Q      81    ox    turned script a  open back rounded, Eng. lot   

 e.    E      69    edje    epsilon  e       open-mid front unrounded, C3, Fr. même    
                                                      [note that e [as in they] is not included]

 'a a   @      64    ago     turned e        schwa, Eng. banana   [the apostrophe in 
                                                      front of any letter makes it schwa, 'e, 'a, 'i]

 'r  r   3      51    her      rev. epsilon    long mid central, Eng. nurse, her, urban   [rr] or [@@]

  i.     I      73     in       small cap I     lax close front unrounded, Eng. kit  [dotless i]

 o     O     79    awe     turned c        open-mid back rounded, Eng. thought, awe, all  

 o'    2      50    oat      ø   /ou/          close-mid front rounded, Fr. deux, Eng. oat, silo  /'u/ without glide

 -     9       57    -        oe ligature      open-mid front rounded, Fr. neuf 

 -     &      38    -       s.c. OE lig.      open front rounded

 u.    U      85    hook    upsilon         lax close back rounded, Eng. foot, hook, put

 -     }     125    -       barred u           close central rounded, Swedish sju

 'u v  V     86    up      turned v /L /     open-mid back unrounded, Eng. strut

 -     Y      89    -       small cap Y      lax [y], Ger. hübsch

    SAMPA    spelling          pronunciation
        i                si                      si
       e              ses                  se
        E               seize                sEz    epsilon
        a               patte                 pat      ?

        A               pâte                  pAt     aa
        O               comme             kOm
        o               gros                  gRo
        u               doux                 du      [deux d'eau  = do]

        y               du                     dy
        2               deux                 d2
        9               neuf                  n9f
        @             justemen           Zyst@ma~

Add diphthongs

[Dr. David Kelley wrote] 
.... Having said that, I am now ready to break any or all the rules -- well,
maybe just one.  Sometimes even the IPA people get us (or themselves) into trouble. 

The IPA symbol for the A in CAT is actually a digraph, even though they push the A and E tight up against each other, and try to pretend otherwise.  [I would say that æ is a ligature. This which would be a less controversial categorization]

And because THEY couldn't come up with anything better, I don't see any reason why we can't use AE to represent the same sound.

And as I mentioned to Ian and Steve recently, the IPA symbol for the AY in PLAY and the EY in GREY is actually just a single symbol (e), not a diagraph as both Ian and I use in our respective spelling systems.  So technically speaking, neither of us should be using the EI digraph.  But then I hear two sounds blended into one another, just as I suppose Ian does, so I think EI is totally appropriate.

[Steve] There are pure e and o vowels that could replace the English /ei/ and /ou/ without misinterpretation.
This is not the same as saying that /ei/ is really e or that /ou/ is really o.
The French e and the Spanish o are pronounced a little differently than our /ei/ and /ou/.

Is Ian ready to say that ei and ou [6} are pure vowels - bringing to total to 14 pure vowels?
 
 

[left] The 14  pure vowels are identical to IPA.  The chart shows more combinations than found in Jones' list of 21 vowels.  iu, aar, air, our are included here [teal]. [right] Notice that each vowel letter represents three sounds.  Usually there are two free vowels and a checked vowel.  U has two checked vowels up and hook.  In a complete alphabet, there are almost as many vowels as there are consonants.

White=unstressed vowel  Yellow =vowel Tan = diphthong,   blue=consonant Green = semi vowel or syllabic consonant

fuff, fuf, fuuf, foof, fauf, faof, fof, foff, 

addrs: www.yahgroups.com/group/saudnspel/files/...

Using Caps and Numbers as Phonograms
 ei can be written A and oa can be written O
c = ae    q = aa     I= ai
e           &           A=ei
i             E = ii Y  Q=oi
q           o=aw      O=&u
w            u           U=yu
v            vr           W=ou

One of the goals is predictable spelling:  othr vthr  cpr

INDEX of SPELLING LINKS http://victorian.fortunecity.com/vangogh/555/Spell/sitemap-l.html

WINGLISH  http://victorian.fortunecity.com/vangogh/555/Spell/winglish.htm

WINGSPAN   http://victorian.fortunecity.com/vangogh/555/Spell/wingspan.htm

New ROMANJI  hawk.hama-med.ac.jp/dbk/new_romaji.html
 Now stored on the egroups server

Yet there is no minimal pair for [ng] in English although this is clearly a phoneme. bank sounds different than baNk where N=[ng].

Has anyone found a minimal pair for [ng] in English?
The usual examples singe [siNj] and sing [siN] and singer/finger [siNer fiNger] 
are not really minimal pairs - removing the N does not change the meaning.

http://www.jbgoodwin.com/area/virtualtour.html#area67

we lesser men fill in the structure - basil blackwell

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